Avoid pathological behavior in OpenBSD's fread.

Bug: https://code.google.com/p/android/issues/detail?id=81155
Bug: 18556607
Change-Id: Idc60976b79610e2202cc42dc393dcb4ca6c42e05

tests/stdio_test.cpp

@@ -813,3 +813,34 @@ TEST(stdio, freopen_CLOEXEC) {
 
   fclose(fp);
 }
+
+// https://code.google.com/p/android/issues/detail?id=81155
+// http://b/18556607
+TEST(stdio, fread_unbuffered_pathological_performance) {
+  FILE* fp = fopen("/dev/zero", "r");
+  ASSERT_TRUE(fp != NULL);
+
+  // Make this stream unbuffered.
+  setvbuf(fp, 0, _IONBF, 0);
+
+  char buf[65*1024];
+  memset(buf, 0xff, sizeof(buf));
+
+  time_t t0 = time(NULL);
+  for (size_t i = 0; i < 1024; ++i) {
+    fread(buf, 64*1024, 1, fp);
+  }
+  time_t t1 = time(NULL);
+
+  fclose(fp);
+
+  // 1024 64KiB reads should have been very quick.
+  ASSERT_LE(t1 - t0, 1);
+
+  for (size_t i = 0; i < 64*1024; ++i) {
+    ASSERT_EQ('\0', buf[i]);
+  }
+  for (size_t i = 64*1024; i < 65*1024; ++i) {
+    ASSERT_EQ('\xff', buf[i]);
+  }
+}